The present invention relates to an improved reflection electron diffractometer and a method for observing microscopic surface structures of a sample by means of reflection electron diffractometry.
The growing demand for higher quality integrated circuits necessitates a more advanced technology of precisely controlling the deposition or expitaxial growth of microscopic metallic films forming an integrated circuit.
For example, micron-sized narrow aluminum or aluminum alloy current-lead films wired in an integrated circuit, if they have their crystalline directions not aligned properly, are liable to get snapped due to the electromigration effected in the films when large currents flow through them or due to the stress-migration originated from stresses often concentrated at the crystal boundaries of the substrate on which are laid the films. In order to check and evaluate integrated-circuits just formed on a wafer and also to establish the conditions that make it possible to produce integrated circuits made free from such trouble, it is necessary to provide with any convenient means for easily observing and evaluating the microscopic inner crystalline structure of the current-lead films and the micron-order near-surface fine crystalline structure of the substrate on which the current-lead films are laid. Obviously, a usual X-ray or electron diffractometer, which uses an X-ray or electron beam with a diameter ranging roughly from 0.1 to 1 mm, is useless for the purpose, because the beam diameter is too large to give microscopic information contained in a region of the order of micron, only capable of giving some information averaged over the region.
A transmission type electron microscope, which makes microscopic crystalline observation possible, is not practical either, because it is inevitably accompanied by a troublesome process of preparing a sample piece taken out from a wafer on which are formed integrated circuits. Further, the necessity of sample piece to be taken out makes it impossible to evaluate the films and substrate under the condition that the wafer with integrated circuits already formed thereon should be kept as it is.
Alternatively, a reflection high energy electron diffraction (RHEED) method may seem applicable to the examination of near-surface crystalline structure. However, this method is not applicable either, because it also gives only some averaged information contained in a wide region spreading over 0.1 to several millimeters in accordance with the diameter of the electron beam used. As to the problem of electron beam diameter, a microprobe RHEED method may be considered to be only a possible means for examining microscopic crystalline structure. Since this method uses as thin a beam as the order of 0.1 micrometer, the size distribution of crystallites is obtained by analyzing the intensity variation of a specific diffraction spot, the intensity variation being observed with the beam made to scan the surface of an objective of obsevation. However, as to the information on the directions of crystallites, the microprobe RHEED method gives only the information on the specific crystallite-faces having a common direction with respect to the incident electron beam, failing to give any information as to the directional distribution of the faces vertical to the above specific crystallite-faces.
As is briefed above, any conventional apparatus or method for crystallographic analysis cannot give enough information to evaluate the microscopic crystalline structure in the films formed on a substrate and in a micron-order shallow depth of the substrate.
In addition to the above-mentioned problems, there is a further need for a method of precisely observing one-atomic layer unevenness on the surface of a semiconductor substrate.
For instance, when gallium arsenide is to be epitaxially deposited on a silicon substrate whose atomic distance is different from that of gallium arsenide, a formed long-ranging gallium arsenide film is often accompanied by lattice defects. Therefore is desirable to make the silicon substrate uneven-surfaced so as to form one-atomic layer steps having their treads sized so as not to be too wide to make gallium arsenide films deposited thereon substantially free from lattice defects. In another case where a super high speed device is manufactured, the silicon substrate is, to the contrary, desired to be as smooth as possible, because even a one-atomic layer step causes electron mobility to be lowered.
To evaluate the smoothness (or unevenness) of the order of one-atomic layer, the previously mentioned microprobe RHEED method is found applicable. However, this method can provide only a general concept about the distribution of surface steps, failing to give the exact information on the surface steps or smoothness, because the method is not only barely capable of giving a low-contrast surface image on a CRT but also is lacking in a means for identifying a region being observed.